Roll to roll thin film vacuum deposition is often used to coat flexible materials such as plastics, metals and polyimide. In typical configurations a material or web substrate is unwound from a payout roll into a processing chamber. After a series of steps are performed, the coated web substrate is wound onto a take-up roll. Many of the deposition technologies that are commonly used require high vacuum conditions, for example, a pressure below 1 Torr. High vacuum deposition technologies include vacuum evaporation and sputtering, often referred to generically as physical vapor deposition. The deposition process induces a heat load on the web, resulting in an increase in temperature. A variety of sources may generate the heat load. For example, the heat load can be due to the heat of condensation of the growing film, a hot surface of the process apparatus that is in the line of sight of the web substrate, and ion or electron currents due to proximity to a plasma during sputtering. In many applications, the increase in the temperature of the web is unacceptable. For example, at high coating speeds and web transport rates, the heat load can cause the web to wrinkle and crease, possibly resulting in permanent damage to the web. As the heat load is typically proportional to the deposition rate, the throughput of the deposition system is severely limited. Consequently, a means to cool the web during deposition can be used to increase the throughput and productivity of the deposition system.
A rotating cooling drum can be used to cool the web during the deposition process. The thermal conductance between the web and the cooling drum affects the ability to control the temperature rise of the web during the coating process, and sets an upper limit to the coating speed. The cooling drum can be used to introduce a gas between the drum surface and the web to increase the rate of heat transfer; however, the gas is introduced in the vacuum region of the system and therefore can adversely affect the deposition process. U.S. Pat. No. 3,414,048 describes a cooling drum that has pistons distributed about the drum surface such that only pistons that are adjacent to the web are activated to provide gas. The other pistons are closed and therefore do not introduce gas into the vacuum environment. However, the cooling drum is mechanically complex as a large number of pistons are required along the surface of the drum to provide a sufficient distribution of the gas between the web and the drum. Thus the cooling drum is expensive and requires significant time to assemble. Moreover, the large number of moving parts can decrease operational reliability.